PEMCFs are current generators whose operating principle, illustrated in FIG. 1, is based on converting chemical energy into electric energy by means of a catalytic reaction between hydrogen and oxygen.
Membrane electrode assemblies or MEAs 1 (also referred to as fuel cell stacks) are the basic elements of PEMCFs. They consist of a polymer membrane 2 and catalyst layers 3, 4 on either side of the membrane which constitute the anode and cathode respectively. Membrane 2 is made using a proton conductive polymer such as Nafion® (Dupont de Nemours), Hyflon® (Solvay) or Dow® (Dow Chemical) (registered trademarks), which are fluorinated polymers.
Membrane 2 is therefore used to separate anode compartment 5 from cathode compartment 6. Catalyst layers 3, 4 generally consist of proton conductive polymers and platinum nanoparticles on carbon aggregate supports. Gas diffusion layers 7, 8 (carbon fabric, felt etc.) are arranged either side of MEA 1 to ensure electrical conduction, uniform distribution of the reactive gases and removal of the water produced. A system of channels 9, 10 located either side of the MEA supplies the reactive gases and removes water and excess gases.
On anode 3, the oxidation of hydrogen on the catalyst produces protons H+ and electrons e−. The protons then pass through polymer membrane 2 before reacting with the oxygen on cathode 4. The reaction of the protons with oxygen on the cathode results in the formation of water and the production of heat.
Extending the service life of PEMCFs is a major issue which affects the use and development of fuel cells for the consumer market. This is why it is now essential to examine and understand the ageing phenomena which affect fuel cell stacks.
The ageing of fuel cells appears to involve several phenomena.
Firstly, numerous scientific studies have demonstrated that the ageing of fuel cells is associated with, among other things, changes in the nanostructure/microstructure properties of the active catalyst layer. Thus, J. P. Meyers and R. M. Darling (J. Electrochem. Soc., 153 (8), A1432, 2006) have reported a marked reduction in the thickness of the active layer on the cathode after several hours of operation (FIG. 2).
This degradation is due to corrosion of the carbon catalyst support caused by the following reaction:C+2H2O⇄CO2+4H++4e−  (2)
The same authors have demonstrated that this degradation is worsened if the fuel cell is subjected to power cycles.
The potential of this reaction (1) is approximately 0.2 V/ENH. Given that the cathodic potential of a fuel cell generally exceeds 0.2 V, this reaction always takes place.
Ideally, the operation of a PEMFC is characterised by the presence of hydrogen which is used as a fuel on anode 3 and air/oxygen which constitutes the oxidant, on cathode 4. These gas exchanges are shown in FIG. 3.
In reality, membrane 2 is not totally impermeable to the gases during operation of a PEMFC. Consequently, a portion of the oxygen moves from cathode 4 to anode 3. This phenomenon is generally referred to as “crossover”. Obviously, the use of fine membranes accentuates this phenomenon.
The oxygen which is present on anode 3 is normally reduced by the hydrogen in the anode compartment. However, during shutdown/start-up phases, power cycles and during water slugging and when the hydrogen supply is switched off, the hydrogen is not sufficient to reach the oxygen. During these phases the oxygen which is still present makes use of other sources of protons, in particular, protons produced by oxidising the cathodic carbon. The oxygen which is present on anode 3 therefore acts as a proton pump which accentuates corrosion of the carbon at the level of cathodic catalyst layer 4 and the reaction (1) is therefore strongly shifted to the right (cf. FIG. 4):C+2H2O⇄CO2+4H++4e−  (2)
Moreover, damage to the carbon support on the cathode causes a loss of catalyst surface area and an increase in the contact resistance between cathode 4 and gas diffusion layer 8. This is one of the factors which causes reduced durability of PEMCFs.
One proposed solution to stop the corrosion of carbon on the cathode involves introducing carbon dioxide (CO2) in the air on cathode 4 and monitoring the quantity of CO2. This technical solution is described in document JP 2006-278190 and is illustrated in FIG. 5. Introducing CO2 shifts reaction (1) to the left and the consumption/corrosion of carbon is therefore slowed down:C+2H2O⇄CO2+4H++4e−  (3)
Alternative approaches to overcoming the problem of carbon corrosion on the cathode have been proposed. They involve, in particular, using more resistant carbon supports such as those described, for example, in the document by T. R. Ralph et al. (ECS Transactions 1 (8) 2006, 67-84).
It should be noted that all these solutions concentrate on interventions on the cathode only.
Other studies have revealed that the ageing of fuel cells is associated with degradation of the proton conductive electrolyte which is largely present in membrane 2 and in active catalyst layer 3, 4 (Schmittinger and Vahidi, J. Power Sources, 180, pp 1-14, 2008).
Thus, as shown in FIG. 6, it has been established, in particular, that the formation of hydrogen peroxide H2O2 and hydroperoxyl radicals within the cell is one of the main causes of electrolyte degradation (Ohma et al., J. Power Sources, 182, pp 39-47, 2008; Mittal et al., J. of The Electrochemical Society, 154(7), B652, 2007). Nevertheless, other chemical reactions can also degrade these polymers.
These compounds are produced by the following reactions which take place on the surface of the anodic catalyst:H2+2M→2(H−M)  (4)(H−M)+O2+2M→(H−M)+2OM→HO2−M+M  (5)(HO2−M)+(H−M)→H2O2+2M  (6)M represents a site on the catalyst.
Equation (4) corresponds to the first stage of the hydrogen oxidation reaction on the anode. This hydrogen adsorption reaction takes place even in the absence of an electric current.
Equation (5) corresponds to reaction of the adsorbed hydrogen with oxygen originating, in particular, from the cathode due to crossover through the membrane in order to form the hydroperoxyl radical adsorbed on catalyst M. This oxygen crossover is at its highest when the fuel cell is open-circuited, the production of hydrogen peroxide is accentuated and ageing is particularly significant under these conditions.
Equation (6) represents the formation of hydrogen peroxide H2O2 due to the reaction of adsorbed HO2 and H.
To sum up, degradation of the membrane and/or ionomer present in the electrodes is caused in particular by the hydrogen peroxide produced on anode 3 due to a chemical reaction at local sites (M) between anodic hydrogen and a portion of the oxygen originating from the cathode due to crossover. Another portion of the oxygen does not react with the hydrogen but causes an anodic oxygen reduction reaction which accelerates the proton pump effect and corrosion of the cathodic carbon as described above.
In practice and in such a case, the hydrogen peroxide attacks the carbon chains and chemical sites which allow the conduction of protons and which are present, for instance, in Hyflon® Ion/Dow (short-chain polymers) or Nafion® (long-chain polymers), the structures whereof are represented below:

Hommura et al. (J. Electrochem. Soc., 155 (1), A29-A33, 2008) and Curtin et al. (J. Power Sources, 131 (½), pp 41-48, 2004) have reported reactions which illustrate the reactivity of a proton conductive polymer with hydrogen peroxide:H2O2→2OH°  (7)Rf—CF2COOH+OH°→Rf—CF2°+CO2+H2O  (8)Rf—CF2°+OH°→Rf—CF2OH→Rf—COF+HF  (9)Rf—COF+H2O→Rf—COOH+HF  (10)Rf represents the chemical composition of the polymer matrix.
Solutions to limit degradation of the actual membrane itself have already been suggested, these include:                Incorporating metallic oxide micro particles inside the membrane and/or electrodes, e.g. particles in the group consisting of alumina, titanium dioxide, zirconium oxide, germanium, cerium or any combination of these materials that ensures decomposition of the H2O2 (WO 2007/108949);        Developing composite membranes with enhanced mechanical properties and chemical stability (see, for example, WO 2005/045976).        
Nevertheless, there still remains a need to develop technical solutions which make it possible to limit the ageing of PEMFC type fuel cells.